Variable resistor control system



Oct. 23, 1956 P. H. ESTES 2,768,274

VARIABLE RESISTOR CONTROL SYSTEM Filed 001;. 27, 1953 Fig.3.

Inventor: I Phillip H; Estes,

y JMK His Attorney.

United States Patent O VARIABLE RESISTOR CONTROL SYSTEM Phillip H.Estes, Schenectady, N. Y., assignor to General Electric Company, acorporation of New York Application October 27, 1953, Serial No. 388,605

13 Claims. (Cl. 219) The present invention relates to control systemsfor use with or which embody a variable resistor, it being understoodthat by a variable resistor I mean an electrical resistor which under agiven set of operating conditions, which may be termed normal operatingconditions, has a high resistance, for example a resistance so high asto be essentially an insulator, and which under another set of operatingconditions, which may be termed abnormal operating conditions, has alower resistance whereby an increase in flow of current through it ispermitted, the increase in flow of current functioning to effectoperation of a relay in the control system. The system may be utilizedin connection with any operating condition which upon change will effecta change in the resistance of the resistor. As examples, a systemembodying the invention may be responsive to a change in temperature, achange in pressure, or a change in light intensity; and the relay whenoperated may cause the operation of a signal, the opening or closing ofan operating circuit, or both.

One application of my invention is to temperature responsive controlsystems for use with heating cable of the type comprising two wires, forexample a resistor wire and a signal wire, separated from each other bya thin layer of a material which at a lower temperature is essentiallyan insulator and a higher temperature becomes an electrical conductorwhich will permit of the flow of significant current, for connecting thecable to a source of alternating current such as the 115 volt source inordinary use. Such cable and its structure is now known. One formwherein the thin separating layer is an organic material such as nylonis disclosed in the patent to Spooner et al. 2,581,212, issued January1, 1952, and assigned to the same assignee as is the instantapplication. The invention has especial utility in such control systemswherein the heating cable is embodied in electrically heated bedcoverings such as blankets and sheets heated by such cable, and this isone application of the invention which I have elected to specificallyillustrate and describe. It is to be understood, however, that this isonly by way of example and that the invention may be used wherever foundapplicable.

An object of the invention is to provide an improved control system foruse, for example, in connecting cable of the type referred to in thepreceding paragraph to an alternating current source, which is simplerin structure than systems used heretofore, quieter in operation, andwhich at the same time is equally reliable in operation.

Other objects and advantages of the invention will be pointed out in andwill appear from the following detailed description of the invention;and for a consideration of what I believe to be novel and my inventionattention is directed to the following specification and to the claimsappended thereto.

According to one form of the invention when used in a bed covering, theresistor wire which is utilized to heat the covering and the signal wireof a cable of the above referred to type are connected to a source ofalternating current by a control system comprising a heating circuitconnected to the resistor wire and a signal circuit connected to thesignal wire, there being in the signal circuit the winding of a directcurrent signal relay to which current is supplied through a rectifier,the direct current relay functioning to control the heating circuit andalso its own circuit.

In the drawing,

Fig. 1 is a diagrammatic view of a bed covering heated by cable of thetype above referred to, the cable being connected to a control systemembodying the invention;

Fig. 2 is a view similar to Fig. 1 showing a modified form of controlsystem embodying the invention;

Fig. 3 is a similar view showing still another form of control systemembodying the invention.

Fig. 4 is a diagrammatic view showing a different application of theinvention.

Referring first to Figs. 1 to 3, the dotted rectangle 4 indicates theoutline of a bed covering having distributed over it in the nowwell-known manner a heating cable of the type referred to above, 5indicating the heating resistor wire of the cable, 6 the signal wire and7 the organic separating layer. The ends of the resistor wire and thesignal wire are indicated as being connected to a plug connector 8, theconnector having terminals 9 to 10 to which the resistor wire isconnected and terminals 11 and 12 to which the signal wire is connected.Also, in Figs. 1 to 3, the dotted rectangle 13 indicates a control boxin which the elements which make up the control system are housed.

Referring now to Fig. l, 14 indicates a plug having terminal prongs 15and 16 for connecting the control system to a source of power, forexample a conventional volt, 60 cycle alternating current domesticcircuit. Heating resistor wire 5 is connected to terminal prongs 15 and16 by way of plug connector terminals 9 and 10, a pair of terminals of aplug connector 17 having terminals for detachable connection withterminals 9, 10, 11 and 12, and conductors 18 and 19. In conductor 19are the contacts 20 of a cycling switch and the contacts 21 of a directcurrent signal relay 22. The cycling switch is shown diagrammatically ascomprising a bimetallic thermostat 23 which carries at its free end oneof the contacts 20, and a heating coil 24 for heating thermostat 23.Heating coil 24 is in circuit conductor 19 and on one side is connectedto the other contact 20. The construction and operation of cyclingswitches of this type is now Well-known. Thermostat 23 operatesalternately to open and close a circuit through heating coil 24 inresponse to heat supplied to it by coil 24 and by the surroundingatmosphere. At a lower temperature when the control is calling for heat,contacts 20 are closed by the flexing of thermostat 23 toward the right.When heated to a. certain extent by heating coil 24 thermostat 23 flexestoward the left to separate contacts 20 to open the heating circuit onboth the heating resistor wire 5 and the heating coil 24. A cyclingswitch of this type is disclosed in the patent to Kearsley, No.2,195,958, issued April 2, 1940.

Relay 22 of which contacts 21 are a part, has its winding 25 connectedin a signal circuit with signal wire 6 which circuit includes ahalf-wave rectifier 26 for supplying direct current to the relaywinding, and the contacts 21 of the relay. This signal circuit may betraced as follows: plug terminal 15, conductor 18, rectifier 26, acurrent limiting resistance 27, conductor 28, connector terminal 12,signal wire 6, connector terminal 11, conductor 29, direct current relaywinding 25, a current limiting resistance 30, conductor 31, and relaycontacts 21 to plug terminal 16. Connected across the terminals of relaywinding 25 is a capacitor 32 for supplying power to the winding duringthe non-conducting half cycles of the It will be noted that both theheating circuit and the signal circuit include relay contacts 21, thesignal circuit serving also as a holding circuit for the relay.

At 33 is an on switch connected in shunt to relay contacts 21 andcurrent'limiting resistance 30 for initially closing a circuit on relaywinding 25 to turn on t e control circuit; and at 34 is an off switchconnectedin shunt to relay winding 25 for turning off the controlcircuit. Switches 33 and 34 are suitable manually'operated switchesbiased to open positions.

In operation, assuming plug 14 has been connected to a source ofalternating current, the user closes on switch 33 to close the signalcircuit through signal relay winding 25 but in shunt to relay contacts21 and current limiting resistance 31). Thiscauses the relay to operateto close the signal circuit through relay contacts 21. The userthen-releases on switch '33 permitting it to open. The relay remainsenergized to hold contacts 21 closed since its winding is in the signalcircuit which includes the relay contacts. When on switch 33 is opened,current limiting resistance 30 is put back into the circuit throughdirect current relay winding 25. This reduces the current flow throughthe relay to a value near the drop out point so that a relatively smallreduction of current flow through the relay will cause it to open ordrop out. With relay contacts 21 closed, the heating circuit for heatingwire 5 may be closed and opened by the cycling switch comprisingthermostat 23 to effect closing and opening of the heating circuit thuseffecting normal operation of the bed covering.

If, when the blanket is on, it should become overheated for any reasonat a point in the vicinity of one end of the cable, for instance at thepoint indicated at A, then due to decre"se in the resistance of layer 7,a path for flow of current between the heating resistor wire 5 and thesignal wire 6 will be formed as indicated by the arrows a. This wouldform a path for flow of current around rectifier 26 and resistance 27 byway of conductor 18, leakage path a, conductor 29, direct current relaywinding 25, resistance 30, conductor 31, and relay contacts 21,

thereby putting alternating current directly on direct current in shuntto relay winding 25 by way of cycling switch contacts thereby reducingthe current flow through winding to cause the relay to drop out, thusopening at relay contacts 21 both the signal circuit and the heatingcircuit. Since the normal current flow through the relay winding is justsuficient to hold it closed, the

relay will open upon a relatively small reduction in such current flow.Should the blanket become overheated at any point between the vicinityof its ends, then the disturbances in the circuit will cause the relayto open due to a combination of the shunting of and the impression ofalternating current on the winding of the direct current relay.

Fig. 2 shows a modified form of control system similar to that of Fig. 1and to which reference numerals corresponding to those used in Fig. lwith the letter a added have been applied to corresponding parts. TheFig. 2 system differs from that of Fig. 1 in that relay winding 25a andresistance 270 are connected in parallel with signal wire 6 instead ofbeing in series therewith, the

capacitor 32a is placed in series with signal wire 6, and

off switch 34a is connected to shunt out the rectifier 26a. The normaloperation of the control system of Fig. 2 and its operation whenoverheating occurs at a pointin the covering is the same asthatdescribed already in connection with Fig. 1. If overheating occursin the vicinity of the upper end of the cable, a path for flow ofcurrent in shunt to the relay winding is established by way of conductor18a, the flow path between wire 5 and 6, and signal wire 6, thusshunting or short circuiting the relay and causing it to drop out; andif overheating occurs in the vicinity of the lower end of the cable apath for flow of current through the relay winding in shunt to therectifier 26a is established by way of relay winding 25a, resistance27a, conductor 28a, the flow path between wires 5 and 6, cycling switchcontacts 2% and relay contacts Zita thus putting alternating currentdirectly on the direct current relay winding to cause it to drop out.

In Fig. 3 is shown another control system embodying my invention. In theFig. 3 construction the signal circuit is similar to that in Fig. l butthe bimetallic cycling thermostat instead of closing and opening theheating circuit directly operates through the intermediary of a directcurrent heater relay 40 capable of quiet operation on half wave currentand having a set of back contacts 31 and a set of front contacts 42. Thewinding of relay &9 is designated 43. In Fig. 3 parts corresponding tothose of Fig. 1 have been designated by the same reference numerals withthe exponent b added. The Fig. 3 modification will be understood readilyby a description of its operation.

Assuming plug 14b has been connected to a source of aiternating current,then when on switch 33b is closed a circuit is established throughwinding 25b of direct current signal relay 22b as follows: plug prong15b, on switch 33b, conductor 45, bimetal 23b, contact 46 (if at thistime the ambient temperature is of a value high enough so that thebimetal is in engagement with contact 46), conductor 47, signal relaywinding 25b, resistance 38b, signal wire 6, conductor 48, relay contacts41, and rectifier 26b to plug prong 161). if at this time bimetal 23b isnot in engagement with contact 46 then the circuit through signalwinding 25]) will be by way of conductor 49 and relay winding 43.However, relays 22b and -30 are so related to each other that while thecurrent flowing in the circuit will be sufiicient to operate relay 2% itwill not operate relay 48. In other words, relay 22b is designed tooperate on a current of a small value not sufficient to operate relay4t). Signal relay 22b closes contacts 21b and establishes the signalcircuit as follows: plug prong 15b, signal relay contacts 21b, signalrelay winding 25b, resistance 30b, signal wire 6, conductor 48, heaterrelay contacts 41, and rectifier 26b to plug prong 16b. This establishesthe holding circuit on signal relay 22b to maintain its contacts 21bclosed, thus holding the signal circuit closed. There is establishedalso a circuit through heater relay 4!) as follows, assuming bimetal 23bstands out of engagement with contact 46 and also cooperating contact 5tplug prong 15b, signal relay contacts 21b, heater relay winding 4-3,conductors 4? and 45, conductor 51, resistances 52 and 53, conductor 61and rectifier 26b to plug prong 16b. Ho vever, resistances 52 and 53have values such that when in series the current flowing through heaterrelay winding 43 is not sulficient to operate it. When bimetal 23b coolsdown it engages contact thus short circuiting resistance 52, i. e.,cutting it out of the circuit through winding 43. The current thenflowing in the circuit is sufficient to cause relay 412 to operate,opening contacts 41 and closing contacts 4-2. The opening of contacts 41puts current limiting resistance 2712, which is connected in shunt tocontacts 41, in the signal circuit thus reducing the current flowthrough direct current signal relay winding 25!) to near the drop outpoint. The closing of relay contacts 42 closes the heating circuitthrough resistor wire 5 as follows: plug prong 3115b, contacts 42,conductor 5'5, resistor wire 5, and conductors 56 and 54 to plug prong16b. It closes also a circuit through thermostat heating coil 24!) asfollows: plug prong 15b, contacts 42, conductor 55, heating coil 24b,and conductor 54 to plug prong 16b. Current is thus supplied to heatingresistor wire 5 and to thermostat heating coil 24b. Thermostat heatingcoil 24b imparts heat to bimetal 23b and after a period of time bimetal23b will move from engagement with contact 50 thus putting resistance 52back into the circuit through heater relay winding 43. This reduces thecurrent through the winding, but such current is still sufficient tohold the relay in. Eventually, bimetal 23b will be heated until itengages its contact 46. This short circuits relay winding 43 by way ofconductor 47, contact 46 and bimetal 23b whereupon the relay drops outopening contacts 42 and closing contacts 41. This opens the circuitsthrough resistor wire and thermostat heating coil 24b. The parts are nowpositioned to repeat the heating cycle.

The signal circuit, as stated, is similar to that of Fig. 1. Itfunctions in the same way to effect operation of signal relay 22b toopen contacts 21b in case of overheating in the cable.

At 60 is a suitable lamp connected across the source to indicate whenthe system is operating.

In connection with the Fig. 3 construction it will be noted that onswitch 33b is connected in shunt to direct current signal relay contacts21b and direct current heater relay winding 43 so that after the onswitch is closed to cause relay 22b to operate to close its contacts2117, a circuit will not be closed on relay winding 43 to efiect closingof the heating circuit at relay contacts 42 until switch 33b opensagain. This means that a holding closed or a jamming in on position ofon switch 33b cannot cause overheating of the bed covering and insuresthat before the heating circuit is put in operating condition, relay 40has operated to open contacts 41 to place resistance 27b in the signalcircuit so as to reduce the current in the signal circuit to the normallow level current value so as to provide full protection againstoverheating.

In Fig. 4 is illustrated a modification of my invention utilizing a formof variable resistor other than that illustrated in Figs. 1 to 3, thecontrol system being shown as operating an indicating signal.

Referring to Fig. 4, wherein the elements of the system are shown asbeing in their operating positions, direct current relay 65 has itswinding 66 connected in a circuit as follows: plug terminal 67 whichrepresents one side of an alternating source, conductor 68, relaywinding 66, rectifier 69, conductor 70, relay holding circuit contacts71, and conductor 72 to the other side 73 of the alternating currentsource. Connected across the terminals of relay winding 66 is acapacitor 74 for supplying power to the winding during thenon-conducting half cycles of the rectifier. A normally open startingswitch 75 is connected in shunt to relay holding contacts 71 for use ineffecting initially the energization of relay winding 66 to cause therelay to pick up.

Connected across the terminals of a rectifier 69 is a variable resistor76 which has the property that under a given normal set of operatingconditions it has high resistance, being essentially an insulator; butwhich under an abnormal set of operating conditions has a lowerresistance thus permitting the flow of current of significant valuethrough it. For example, it may be in the nature of a thermistor, theresistance of which changes with changes in its temperature or it may bein the nature of a piezoelectric crystal, the resistance of which varieswith changes in pressure applied to it. The variable resistor is locatedwhere it is subjected to the operating condition to be controlled. As anexample, in the case of a thermistor, it would be located where it wouldbe subjected to the temperature of the apparatus or device in connectionwith which the control system was used.

As shown in Fig. 4, the circuit through direct current relay winding 66is closed, the relay being kept energized by rectified alternatingcurrent; and as long as variable resistor 76 is subjected to normaloperating conditions of that in connection with which it is used nosignificant electric current will flow through the variable resistor.However, in case the variable resistor is subjected to abnormaloperating conditions whereby its resistance is reduced permitting of theflow through it of electric current of significant value, then thewinding 66 will have applied to it alternating electric current to causethe relay to drop out.

The relay when it drops out may effect the opening or the closing of anelectric circuit to perform any desired operation. For example, it mayeffect the opening or the closing of a power or work circuit. In thepresent instance the relay is shown as being provided with a pair ofcontacts 77 which are bridged when the relay drops out to close acircuit through a signal lamp 78. This circuit is to be taken asrepresenting a power or work circuit.

An important advantage of my control system resides in its quietness inoperation. Heretofore, as far as I am aware, signal systems of thegeneral type to which my invention relates have utilized alternatingcurrent relays and While such relays are satisfactory from a functioningstandpoint they have the disadvantage, especially objectionable in thecase of bed coverings, of being noisy in operation due to inherentalternating current hum. My improved system wherein I utilize a directcurrent relay completely overcomes such disadvantage. At the same timemy system is relatively simple in structure, reliable in operation andcapable of being manufactured at low cost.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, a length of cable comprising a heating resistor wireand a signal wire separated from each other by a layer of material whichat a lower temperature is essentially an electrical insulator and at ahigher temperature becomes an electrical conductor which will permitflow of significant current, and a control system for connecting thecable to an alternating current source comprising a heating circuitconnected to the resistor wire, a direct current relay having contactscontrolling the heating circuit, a signal circuit including a half Waverectifier connecting the winding of the direct current relay to thesignal wire, a capacitor connected in shunt to the relay winding, astarting switch for initially closing a circuit through the winding ofthe relay to energize it to eifect the closing of its contacts, andmeans controlled by the starting switch which maintains the heatingcircuit open as long as the starting switch is in closed position.

2. In combination, a length of cable comprising a heating resistor wireand a signal wire separated from each other by a layer of material whichat a lower temperature is essentially an electrical insulator and at ahigher temperature becomes an electrical conductor which will permitflow of significant current, and a control system for connecting thecable to an alternating current source comprising a heating circuitconnected to the resistor wire, a direct current relay having contactscontrolling the heating circuit, a signal circuit including a half waverectifier connecting the winding of the direct current rclay to thesignal wire, a capacitor connected in shunt to the relay winding astarting switch for initially closing a circut through the winding ofthe relay to energize it to effect the closing of its contacts, andcircuit closing means for the heating circuit including a connectionwith said starting switch to maintain the heating circuit open as longas the starting switch is closed.

3. A control system for connecting to an alternating current source aheating cable of the type adapted for use with an electric bed coveringor the like and including a signal wire and a heating resistor wireseparated from each other by a layer of material which at a lowertemperature is essentially an electrical insulator and at a highertemperature becomes an electrical conductor permitting flow ofsignificant current, comprising a heating circuit including a heatingcircuit relay for opening and closing the connection to the resistorwire, a cycling switch controlling operation of said relay, a directcurent relay having contacts for controlling the heating circuit, and asignal circuit for connection to the signal wire comprising the windingof the direct current relay, a half wave rectifier in series with thedirect current relay Winding and a capacitor in shunt to the relaywinding.

4. A control system for connecting to an alternating current source aheating cable of a type adapted for use ing circuit relay and therebyopen and close said heating circuit, a direct current relay havingcontacts for controlling the heating circuit, and a signal circuit forconnection to the signal wire comprising the winding of the directcurrent relay, a half wave rectifier in series with the direct currentrelay'winding and a capacitor in shunt to the direct current relaywinding.

5. In combination, a length of cable comprising a heating resistor wireand a signal wire separated from each other by a layer of material whichat a lower temperature is essentially an electrical insulator and at ahigher temperature becomes an electrical conductor which will permit theflow of significant current, and a control system for connecting thecable to an alternating current source comprising a heating circuitconnected to the resistor wire, a direct current relay having contactscontrolling the heating circuit, a signal circuit connecting the windingof the direct current relay to the signal wire comprising a rectifier,the relay contacts, a current limiting resistance in series with thewinding, a starting switch, circuit means for connecting the startingswitch to the relay winding for closing a circuit through the relaywinding in shunt to the relay contacts and through the current limitingresistance for initially energizing the relay to efiFect closing of itscontacts, and means controlled by said starting switch for maintainingthe heating circuit open while the starting switch is in closedposition.

6. The combination with an electric bed covering or the like havingdistributed thereover a cable comprising a heating resistor wire and asignal wire separated from each other by a layer of material which at alower temperature is esentially an electrical insulator and at a highertempearture becomes an electrical conductor which will permit the flowof significant current, of a control system for connecting the cable toan alternating source comprising a heating circuit connected to theheating resistor wire, a heating relay having contacts in the heatingcircuit, a cycling switch, a cycling circuit connecting the cyclingswitch to the winding of said heating relay whereby the cycling switcheffects operation of said relay to control the heating circuit, a directcurrent signal relay having contacts which control the cycling circuit,a signal circuit connected to the signal wire comprising the winding ofsaid signal relay, contacts of the signal relay, and a rectifier inseries with the signal relay winding, whereby said cycling switch caneffect operation'of said heating relay to close the heating circuit onlywhen said signal circuit is closed to hold the contacts of the signalrelay closed.

77 The combination defined by claim 6 wherein there is provided an onswitch and a starting circuit for the on switch connected to .the signalrelay winding in shunt to the heater relay winding whereby the heaterre- 8 lay winding cannot be energized to close the heating circuit whilethe on switch is in closed position.

8. In combination, a length of cable comprising a heating resistor wireand a signal wire separated from each other by a layer of material whichat a lower temperature is essentially an electrical insulator and at ahigher temperature becomes an electrical conductor which will permit ofthe flow of significant current and a control system for connecting thecable to an alternating current source comprising a heating circuitconnected to the resistor wire, a relay having contacts controlling theheating circuit, a signal circuit responsive to flow of current betweenthe resistor wire and the signal wire connecting the winding of therelay to the signal wire, a starting switch connected to the relaywinding for closing a circuit through the winding for initiallyenergizing it, and means controlled by the starting switch whichmaintains the heating circuit open as long as the starting switch is inclosed position.

9. An over-temperature protection system for an electrically heateddevice having a heating resistor comprising a direct current relay and arectifier defining first and second circuit elements connected in seriesacross a source of alternating current power, a capacitor connected inshunt to said direct current relay, power being supplied continuously tosaid circuit elements when the system is in operation, said heatingresistor being in electrical parallel with said circuit elements, avariable impedance continuously associated with said heating resistor tobe sensitive to the temperature thereof and having a negativetemperature coefficient of impedance, said variable impedance-beingconnected in electrical parallel with one of said circuit elements andin series with the other of said circuit elements, and a work circuitincluding said heating resistor controlled by said relay.

10. An over-temperature protection system for an electrically-heateddevice having a heating resistor comprising a direct current relay and arectifier defining first and second circuit elements connected in seriesacross a source of alternating current supply, power being suppliedcontinuously to said circuit elements when the system is in operation,said heating resistor being in electrical parallel with said circuitelements, a variable impedance continuously associated with said heatingresistor to be sensitive to the temperature thereof and having anegative temperature coefiicient of impedance, said variable impedancebeing connected in electrical parallel with one of said circuit elementsand in series with the other of said circuit elements, and a workcircuit including said heating resistor and said circuit elementscontrolled by said relay.

11. An over-temperature protection system for an electrically-heateddevice having a heating resistor comprising a direct current relay and arectifier defining first and second circuit elements connected in seriesacross a source of alternating current supply, power being suppliedcontinuously to said circuit elements when the system is in operation,said heating resistor being in electrical parallel with said circuitelements, a variable resistor sensing the operating temperature of thedevice and which when subjected to normal operating conditions isessentially an insulator and when subjected to an abnormal operatingcondition becomes a conductor to permit flow of significant current,said variable resistor being connected in electrical parallel with oneof said circuit elements and in series with the other of said circuitelements, and a work circuit including said heating resistor controlledby said relay.

12. An over-temperature protection system for an electrically-heateddevice having a heating resistor comprising a direct current relaywinding and a rectifier defining first and second circuit elementsconnected in series across a source of alternating current, power beingsupplied continuously to said circuit elements when the sys-' tem is inoperation, said heating resistor being in electrical parallel with saidcircuit elements, a variable resistor sensing the operating temperatureof the device and which when subjected to normal operating conditions isessentially an insulator and when subjected to an abnormal operatingcondition becomes a conductor to permit flow of significant current,said variable resistor being connected in electrical parallel with oneof said circuit elements and in series with the other of said circuitelements, and a work circuit including said heating resistor and saidrelay winding controlled by said relay winding.

13. The combination defined by claim 12 wherein said 10 work circuitincludes contacts operating when closed as a holding circuit for saidrelay winding.

References Cited in the file of this patent UNXTED STATES PATENTS Re.20,317 Suits Mar. 30, 1937 2,290,909 Hettenhausen July 28, 19422,434,601 Taylor Jan. 13, 1948 2,441,984 Armstrong May 25, 19482,482,820 Wolfson Sept. 27, 1949 2,565,478 Crowley Aug. 28, 19512,592,525 Huck Apr. 15, 1952

